In the seismic exploration industry, it is useful to have a plurality of sensors to transform mechanical earth movement into electrical signals capable of further analysis. These sensors include vertically oriented vibration sensors, called vertical geophones, horizontally oriented vibration sensors, called horizontal geophones, and differential-pressure sensors, called hydrophones. These sensors are used to detect sound waves that are bounced off reflecting sub-strata in a manner well known in the art, and as they detect different aspects of the reflected sound wave it is sometimes useful to use them in conjunction. The method of deploying the cable-sensor system typically leaves the orientation of the sensor packages indeterminate, so it is necessary to insure that any orientation requirement of the internal sensors be otherwise accommodated. Hydrophones as pressure devices are intrinsically not position sensitive. However, the geophones are gravity dependent. Therefore they are typically gimbal-mounted to insure the vertical and horizontal geophones are truly properly oriented relative to the earth. Due to the depths and electrical nature of the geophones, the gimbal-mounted geophone(s) are contained in a protective pressure housing. These sensor packages, which may typically contain a hydrophone, two horizontal geophones, and one vertical geophone tend to be relatively large and massive compared to the cross-section of the host cable used to transmit the data back to the recording vessel.
Previous methods of mounting sensor packages to ocean bottom cables have focused on protecting the sensor package from the deployment and retrieval hardware. They have ranged in complexity from simply taping the sensors to the cable, to using large adhesive-lined heatshrink tubing to completely cover the package and its section of the host cable, to specialized molded parts enclosing the sensor package that would attach to the main cable with steel bands. Each method had its advantages and disadvantages, but all shared a fundamental shortcoming. The sensor package is mounted sidesaddle on the main cable, with the result that the mechanical coupling of the sensors to the ocean floor depended on the rotation of the cable at the sensors location. All rotations of the sensor where the sensor is not in direct contact with the seafloor, other than directly above the cable, result in all vibrations being transmitted through the effective torsional spring of the main cable. This adds an additional frequency dependent transfer function to the sensor""s data, with the specific transfer function varying from location to location. Also note that these orientations would be a noise source were they to start oscillatory vibration due to any water currents or wave movement.
A remedy for this problem is to emplace the sensor package""s cylindrical axis coaxial to the cable""s axis, thereby removing any possibility of spring-mass interaction. The difficulty with doing this is that this method requires the cable""s axial strength member to be severed to create room, and the surrounding electrical wires bird-caged around the sensor. This method weakens the cable unless extraordinary steps are taken in the sensor to carry the tensile load, and if a method can be implemented to carry the tension it is very expensive and labor intensive to produce.
The present invention addresses the need for the sensor package""s coupling to be rotation orientation independent while maintaining the integrity of the host cable, and rugged enough to survive the typical rigors of the average set of cable handling equipment. It also is easy to implement so as to reduce the construction labor costs.
A sensor housing shroud for mounting seismic sensors to a cable is provided which comprises a pair of spaced apart endshot anchor cores bonded to the jacket of the cable, an overmold endshot anchor bonded to each anchor core, an organizer coaxially aligned with and attached about the cable for holding the seismic sensors and their balance rods, an end piece attached about each overmold endshot anchor, and a substantially cylindrically symmetrical outer shell enclosing the organizer and coupled to the end pieces, wherein the housing shroud has a substantially balanced rotational moment. The organizer may be further adapted to hold balance rods, pingers, and/or other seismic oceanographic sensors and their associated balance masses. The end pieces may be attached to flat surfaces on the respective overmold endshot anchors. The sensor housing shroud may include an organizer clamp for securing components to the organizer. In a preferred embodiment, the anchor cores and anchors are made of the same plastic material as the cable jacket and are chemically bonded together to form a monolithic structure. The shell for the sensor housing shroud may be constructed of removable pieces held together by one or more banding straps. The shell preferably is composed of hermaphroditic pieces and uses a tenon and mortise arrangement. The center line of the sensor housing shroud is substantially the same as the center line of the cable. Preferably, each mass held in the organizer is substantially balanced by an offsetting mass radially opposite the cylindrical axis of the cable.
In another embodiment of the invention, a seismic sensor shroud package having a substantially balanced rotational moment is provided which comprises a pair of spaced apart endshot anchor cores bonded to the jacket of a seismic cable, an overmold endshot anchor bonded to each anchor core, a tapered end piece attached about each overmold endshot anchor, a four component seismic sensor attached to an organizer, the organizer being coaxially aligned with and attached about the cable, and a substantially cylindrically symmetrical outer shell coupled to the end pieces and enclosing the seismic sensor, wherein the cylindrical axis of the seismic sensor shroud package is coaxial to the axis of the cable. The seismic sensors for the sensor housing shroud or seismic sensor shroud package described above may comprise a combination of gimbal mounted velocity sensors, oriented to one or more of the three axes, and may also include one or more pressure sensors.